Lyme disease, caused by the tick-borne spirochete Borrelia burgdorferi, results in a persistent, multisystemic infection. Little is known about the virulence and immune evasion of B. burgdorferi, but it has been shown to bind certain cell-surface receptors termed integrins. Integrins are divalent cation-dependent heterodimers that normally mediate cell to cell and cell to extracellular matrix interactions. P66 is a B. burgdorferi outer surface protein that was identified as an integrin ligand, and so is likely to participate in the ability of the bacterium to infect its mammalian host. Aim I of this proposal includes the analysis of site-directed mutants in a region of p66 thought to be important for binding to integrins. These mutants will be compared with wild-type protein and intact B. burgdorferi to identify the specific amino acid residues important for integrin binding. In Aims 2 and 3, biochemical and genetic approaches will be taken to identify B. burgdorferi proteins that regulate p66 expression. The pattern of p66 expression in different environments is unique among the B. burgdorferi proteins studied to date. Two aspects of B. burgdorferi virulence, namely integrin-ligand interaction and regulation of gene expression, will therefore be illuminated by this work.